The specific aims of this proposal are to use resistance to Teniposide (VM-26) as a tool with which to identify cellular determinants of cytotoxicity to the drug. Similar studies of resistance have contributed significantly to understanding the pharmacologic interactions between a drug and tumor cells. For the work purposed, a spectrum of twelve L1210 lines was selected with a 1,200-fold variation in sensitivity to VM-26. The effects of resistance on the cellular levels and disposition of VM-26 will be determined by monitoring influx, efflux, steady-state levels, metabolic products and cellular retention of the drug. Preliminary observations with a silicone oil method indicate that resistance is accompanied by changes in the transport of VM-26 and in the steady-state levels of drug. These studies will be extended to the entire spectrum and if substantiated, to electrophoretic studies of the glycoproteins in the cell membrane, as well as to HPLC studies of the cellular disposition of the drug. An increase in doubling time correlated with increased resistance to VM-26. Since VM-26 blocks cell division at the G2 phase, it is essential to know if the increase in doubling time relates to a prolongation of the G2 phase or to a more general phenomenon. Resistance to VM-26 is a somatic mutation and is associated with a homogeneous staining region (HSR) on G-banded chromosomes. Resistant cell lines will be karyotyped for other abnormalities and for a correlation between the size of the HSR and sensitivity to VM-26. In addition, two-dimensional gel electrophoresis will be used to search for translational expression of the HSR. Drug-induced lesions in cellular DNA are expressed as single-strand breaks on alkaline sucrose gradients. The appearance and disappearance of these potentially oncolytic lesions will be tested for a correlation with other changes associated with resistance. Results from the purposed studies of resistant cell lines are expected to indicate critical determinates for innate sensitivity to the drug and for use of the drug with other oncolytic agents.